Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover the safety profile (readOnlyHint, idempotentHint, openWorldHint=false), but the description adds real behavioral detail beyond them: inclusive endpoints, a defined degenerate case ('Returns 0 if start is after end'), and a hard limit of 3,660 days. It does not explain how the calendar/override parameters interact, which keeps it below a 5.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The lead sentence front-loads the core operation and the second and third sentences cover the two edge cases efficiently. It loses a point only because the full text is duplicated verbatim in Japanese, doubling length without adding information for a single-language reader.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a 5-parameter tool with 100% schema coverage and no output schema, the description supplies the key semantics an agent needs: what is counted, inclusive boundaries, the degenerate result, and the maximum span. The interaction between calendar and the two override arrays is left entirely to the schema, which is a minor gap given full coverage.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema already documents start, end, calendar, extra_holidays, and extra_business_days with format and enum semantics. The description adds nothing parameter-specific beyond restating the start/end relationship, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource ('Count business days from start to end') plus a scope qualifier ('inclusive of both ends'), which lets an agent distinguish it from siblings like add_business_days and check_business_day. It does not, however, explicitly name or contrast those siblings.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is implied by the verb 'count' within a date range, and boundary behavior is stated, but the description never says when to prefer this over adjust_to_business_day, check_business_day, or list_holidays. No exclusions or alternative-selection criteria are given.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.